In order to clarify the precursors of trihalomethane (THM) in tap water, humic substances and other dissolved organic matter (DOM) in Lake Biwa and Yodo rivers were investigated to elucidate their behavior and contribution to the THM formation potential. The hydrophobic and hydrophilic fractions of DOM were isolated using XAD-7HP resin, and then the THM formation potential with the chlorination of these fractions was examined. Hydrophobic acids, such as humic substances, were 30∼60% of DOC in Yodo rivers and about 25% in the northern basin of Lake Biwa. The contribution of hydrophobic acids, such as humic substances, to the THM formation potential was about 70% in Yodo rivers, while it was about 30% in the northern basin of Lake Biwa, which was almost same as the contribution of hydrophilic DOM to THM formation potential. Hydrophilic neutral DOM was 15∼20% of DOC in Lake Biwa, and its contribution of THM formation potential was about 40%, which was larger than those of hydrophobic acids and hydrophilic DOM. The THM formation potential per organic carbon unit of hydrophilic fractions in the northern basin of Lake Biwa were two-times that in rivers. These results suggest that the kinds of hydrophilic DOM in Lake Biwa may be different from those in rivers.